1. Field of the Invention
This invention relates to a variable attenuator for fiber optic transmission cables, and in particular to a variable attenuator of the type in which attenuation is achieved by varying an air gap between ends of two optical fibers. A cam arrangement is included to achieve a linear response between turning of an actuator and attenuation of an optical signal transmitted through the gap between the fiber optic transmission cables.
2. Description of Related Art
Attenuators are used to regulate the strength of an optical signal in a fiber optic transmission system. It is desirable to maintain the strength of an optical signal within a certain range to avoid receiver saturation, to compensate for variable distances of various receivers from the source, or to compensate for aging or other changes in the system.
The present invention involves a variable attenuator that is in the form of a fiber optic coupler arranged such that a controllable air gap exists between the ends of the fibers in the transmission cables coupled by the coupler. By mechanically varying the air gap length, the amount of attenuation can be controlled. Examples of variable attenuators of this type are disclosed in U.S. Pat. Nos. 4,145,110 and 5,066,094. A similar arrangement, involving interception of the air gap by a screw rather than direct adjustment of the air gap length is disclosed in U.S. Pat. No. 5,734,778.
FIG. 1 illustrates one such conventional variable attenuator of the type in which the air gap length in varied by turning a screw, and which is available from Amphenol Corporation, Fiber Optics Products Division, Lisle, Ill. The variable attenuator couples two transmission cables 1,2, each of which is terminated in conventional fashion by: (1) stripping the respective cable jackets 3,4, and strength members 5,6 to expose fiber buffers 7,8; (2) further stripping portions of the fiber buffers 7,8 to expose the fibers and inserting the exposed fibers in alignment ferrules 9,10; (3) polishing ends of alignment ferrules 9,10 so that the ends of the exposed fibers are flush with the facing surfaces of the alignment ferrules; (4) capturing the alignment ferrule holding members 11,12 and bias springs 13,14 within externally threaded front portions 15,16 of rear bodies 17,18 by securing the front portions to internally threaded rear portions 19,20 of front bodies 21,22; (5) crimping exposed portions of strength members 7,8 between rear portions 23,24 of rear bodies 17,18 and crimp ferrules 25,26; and securing boots 27,28 to the rear bodies 17,18.
Front body 21 of the first transmission cable is arranged in conventional fashion to support a coupling nut 29, but slide body 22 of the second transmission cable termination has been modified to be cylindrical in shape and to include external threading for cooperation with an internally threaded thumb wheel 30. Thumb wheel 30 includes a collar 31 that is captured between a front housing 32 and a rear housing 33 which together form the attenuator housing upon threading of rear housing 33 onto an externally threaded extension 34 of front housing 32, and as a result thumb wheel 30 can rotate relative to front housing 32 but cannot move axially. On the other hand, slide body 22 is free to slide axially within front housing 32, causing alignment ferrule 10 to also move axially in response to rotation of thumb wheel 30. Rotation of slide body 22 is prevented by a dowel 35 extending inwardly from front housing 32 and slidably fitted in a slot in slide body 22, while an attenuation spring 36 captured between the front housing 32 and slide body 22 eliminates tolerances between the threading of the thumb wheel 30 and slide body 22.
Alignment of ferrules 9,10 is provided by an alignment sleeve 37 fitted in a holder 38, which in turn is captured between front body 32 and an adapter 39 upon threading of adapter 39 into the front housing. Extending from adapter 39 is a cylindrical coupling section 40 to which the coupling nut 29 is secured to thereby fix cable 1 relative to the housing. In order to vary the length of the air gap between the ends of ferrules 9,10, it is simply necessary to turn thumb wheel 30, causing corresponding axial movement of slide body 22 and ferrule 10 relative to the fixed position of ferrule 9 and front housing 32.
A problem with attenuators of the type illustrated in FIG. 1 is that the relationship between air gap length and attenuation is non-linear, making it difficult to predict the attenuation that will result for a given number of turns of the thumb wheel 30. For example, adjustment of the attenuation from 10 dB to 15 dB might require 1.3 turns of the wheel, while adjustment of the attenuations from 20 dB to 25 dB might require 0.8 turns of the thumb wheel. Adjustment of the attenuation would be much easier to achieve if each turn of the thumb wheel resulted in the same variation in attenuation, i.e., if the relationship between turns of the wheel and attenuation were linear, but such a linear relationship is not possible in conventional screw arrangements of the type illustrated in FIG. 1 due to the non-linear relationship between the air gap length and attenuation of the light signal across the air gap.
A second problem with attenuators of the type illustrated in FIG. 1 is that the thumb wheel or adjustment screw can easily be turned beyond the minimum and maximum attenuation points, causing damage to the attenuator.
The present invention solves these problems by replacing just five parts of the conventional variable attenuator mechanism illustrated in FIG. 1. The five parts are the slide body 22, thumb wheel 30, rear housing 33, a wave washer (not shown) which fits between the slide body 22 and front housing 32, and a Teflon.TM. washer (not shown) which fits between the thumb wheel 30 and rear housing 33. These five parts are replaced by a modified slide body, a cam body, a worm and worm gear mechanism, and a modified rear housing which cooperate to cause the air gap length to be adjusted in such a way that each turn of an actuator will result in a proportional change in the attenuation, as will be described below, thereby simplifying the adjustment procedure without an undue increase in complexity of the attenuator. The cam surface is arranged such that the position of the slide body will reset when the actuator is turned beyond the maximum or minimum positions, making it impossible to damage the attenuator by over-turning of the actuator.